(1) Field of the Invention
This invention relates generally to a front-end circuit of an electro-optical sensing device and relates more specifically to accommodate an infrared pulse of a light receiving diode (LRD), receiving transmitted light from a light source, in presence of ambient current (ambient light in an LRD) for a rain-sensing system. This performs measurements of the attenuation of reflected infrared LED light due to increased scattering of light by rain.
(2) Description of the Prior Art
The usage of infrared senders and receivers is getting more and more popular and is known for numerous different applications and includes a transmitter (such as an infrared light emitting diode (LED)), which transmits light in the infrared range, and an optical receiver (phototransistor or photodiode or photo resistor), which converts the optical infrared signal into a proportional electrical signal.
Rain sensing systems should fulfill the following properties:    1. Providing a 14-bit dynamic range and 250 kHz output rate for accommodating an infrared pulse with an adequate conversion time in the order of 32 μsec.    2. Providing a resolution of 14-bit leading to an equivalent dynamic range about 160 dB of the infrared photocurrents.    3. Providing an ambient light compensation having resolution from e.g. 15 mA down to 1 μA with an accommodating time of about 50 μsec.    4. Providing an infrared current to ambient current ratio maximized by limiting the photocurrent bandwidth and increasing an analog-to-digital converter resolution.    5. Improvement in signal-to-noise ratio provided by correlated double sampling of the sequential measurements from the compensated ambient current and an infrared current pulse.    6. In summary, a high resolution, wide dynamic range, power efficiency and low cost are required
Prior art systems do not cover completely these requirements.
Solutions dealing with photocurrent front-end systems including ADCs are described in the following patents:
U.S. patent (U.S. Pat. No. 7,019,321 to Kokuryo et al) proposes a rain sensing device that can estimate the size of raindrops or the like without a large amount of hardware resources, and its sensing method, and a wiper controlling apparatus using the same. In a sensing device, light emitted from a light-emitting member is introduced to a transparent plate, reflected by a sensing surface of the transparent plate and then received by a photo-detector, thereby detecting a condition of the sensing surface.
U.S. patent Publication (US 2009/0121889 to Lin et al.) discloses a proximity sensor including a driver, a photo-diode (PD) and an analog-to-digital converter (ADC). The proximity sensor can also include a controller to control the driver. The driver selectively drives a light source, e.g., an infrared (IR) light emitting diode (LED). The PD, which produces a current signal indicative of the intensity of light detected by the PD, is capable of detecting both ambient light and light produced by the light source that is reflected off an object. The ADC receives one or more portion of the current signal produced by the PD.
U.S. patent (U.S. Pat. No. 5,929,982 to Anderson) describes an active avalanche photo-diode (APD) and a gain control circuit for use in an optical receiver including a bias generator for varying the bias on a variable gain APD in response to bias control values generated by a controller. The controller receives the output of the optical receiver and determines the system noise of the receiver for the various bias control values. The system noise is compared to a threshold value for establishing the optimum bias for optimum gain of the APD. The gain control circuit is useable in an optical receiver in an optical time domain reflectometer (OTDR) for increasing the dynamic range of the OTDR. The optical receiver includes an amplifier section for amplifying the output of the APD and an analog-to-digital converter for receiving and converting the amplified output of the amplifier section to digital values representative of an optical input received at the APD.